Update esp_mqtt_client_subscribe() macros

[valesnikov]

Change "char *" to "const char *" in esp_mqtt_client_subscribe generic macros. Without these changes, functions passing "const char *" are not compiled

/home/fl/esp/idfmaster/components/mqtt/esp-mqtt/include/mqtt_client.h:464:84: error: '_Generic' selector of type 'const char *' is not compatible with any association
  464 | #define esp_mqtt_client_subscribe(client_handle, topic_type, qos_or_size) _Generic((topic_type), \
      |                                                                                    ^

[euripedesrocha]

Hi @faceleft, thanks for the fix.
Could you modify to add an association for the const char * type and keep the char *? This way we cover both cases.

It would also be nice if you could add a more descriptive commit message title.

[valesnikov]

Done